(1) Field of the Invention
The present invention relates to a projection exposure apparatus, and in particular relates to an projection exposure apparatus which is characterized by a controlling method of its focusing system.
(2) Description of the Related Art Conventionally, in the field of the manufacture of semiconductor devices by utilizing the photolithography, projection exposure apparatuses have been used which uses an optical system to produce a pattern image of a mask or reticle on the surface of a photosensitive substrate such as a wafer or glass substrate etc. with a photosensitive material such as a photoresist applied thereon. Since these projection exposure apparatuses typically use a projection optical system with a large numerical aperture (N.A.) and a small depth of focus, the apparatus needs to have a mechanism of bringing the wafer surface into a focal plane of the projection optical system in order to transfer a minute circuit pattern with high resolution. Therefore, the projection exposure apparatus has a focusing system for bringing the photosensitive substrate surface into a range of the depth of focus of the projection optical system. This focusing system is composed of a focus/leveling detecting part for detecting the position and inclination of the photosensitive substrate surface with respect to the optical-axis direction (Z-direction) of the projection optical system and an adjusting mechanism (to be referred to hereinbelow as a ZT-stage) for adjusting the position and posture of the photosensitive substrate surface based on the detected height and inclination.
Conventionally, so-called step-and-repeat type projection exposure apparatuses have been used mainly in the field. On the other hand, one chip of a semiconductor device tends to be made large-scale, this demands that a greater pattern should be projected on the wafer for exposure. To meet this demand, a projection exposure apparatus of step-and-scan type has been devised which scans the reticle and wafer in synchronism relative to the projection optical system, thus making it possible to effect an exposure of a greater range of the shot area than that of the valid exposure field formed by the projection optical system. In the step-and-scan exposure apparatuses, a pattern on the reticle is illuminated as a slit-shaped area by a light beam which is defined by blinds in illuminating system. The illuminated area on the reticle pattern is termed `illumination field`. On the other hand, the illuminated field on the wafer, that is, a field formed on the wafer in conjugate with the illumination field through the projection optical system is termed `exposure field`. The exposure field is also slit-shaped in the step-and-scan exposure apparatus. In contrast to these terms, a large area on the wafer (photosensitive substrate) into which the whole pattern image on the reticle is projected successively is called as `exposure area`.
In the thus described projection exposure apparatus of step-and-scan type, it is necessary to move both the photosensitive substrate and the reticle in synchronism with each other as to their positions, at a substantially constant velocity which is in conformity with the energy amount to be given, in order to expose a certain area of the photosensitive substrate surface with the pattern of the reticle. To achieve this, a stage (XY-stage) with the photosensitive substrate mounted thereon is started to run from the position prior to the region where exposure is to be made (an approach run), so as to be synchronized with a reticle stage with the reticle mounted thereon during the approach run. Thereafter, when the area to be exposed on the photosensitive substrate reaches the exposure position, exposure will be made. As to the focusing operation, a search of the focal position during the approach run and a shift of the photosensitive surface to the focal position, are also effected as a part of the above procedures. Then, the focusing operation is carried out by following the surface of the moving photosensitive substrate. Typically, in this focusing operation, a focusing action with respect to the optical-axis (Z-axis) direction of the projection optical system is carried out simultaneously with a focusing action of adjusting tilt with respect to X- and Y-axes (leveling operation), in order to minimize the difference between the exposure field of the photosensitive substrate surface and the focal plane of the projection optical system.
In the projection exposure apparatus of step-and-scan type, because the photosensitive substrate to be exposed moves during exposure, adjustment of the photosensitive substrate surface in the exposure field to the focal plane of the projection optically system is successively and repeatedly made. Further, as compared to the projection exposure apparatuses of en-block exposure type such as a stepper, the illuminated area field is narrow therefore it is necessary to drive the ZT-stage at a greater velocity if the above successive adjustment is to be made.
Now, explanation will be made on the driving velocity of the ZT-stage by considering a case in which a surface evenness of the photosensitive substrate is relatively large, for example, in which the photosensitive substrate is scanned in an exposure field (the illuminated area of the substrate) of 8 mm in width in the scan direction and the height difference of the surface in this distance is assumed to be 0.5 .mu.m. In this case, while the substrate is scanned for 8 mm, the surface moves up or down by 0.5 .mu.m with respect to the height direction (z-direction), this can be represented by a tilt angle of 0.5 .mu.m/8 mm=about 62 .mu.rad. Here, if the scan velocity of the photosensitive substrate is 80 mm/sec, the photosensitive substrate passes through the exposure field for 0.2 sec. Accordingly, the leveling operation must be done at a rate of 62 .mu.rad/0.1 sec=620 .mu.rad/sec. The equivalent distance between driving pivots of the ZT-stage for effecting the leveling operation is about 250 mm at maximum for an 8 inch wafer stage, therefore the driving speed in the Z-direction at the pivot positions is about 250 mm.times.620 .mu.rad/sec=155 .mu.pm/sec. On the other hand, the control quantity of the focusing control in the z-direction should adequately be estimated within about 0.5 .mu.m/0.1 sec=5 .mu.m/sec. Therefore, the ratio of the control quantities between the leveling and focusing operations is about 30:1.
This ratio constrains the relation between the resolution of velocity instructions and the maximum velocity in the velocity control system within a certain range. To give velocity instructions to a velocity control system of analog control, a 12 bit D/A converter is often used. In this case, the ratio between the resolution of the velocity instruction value and the maximum value of the velocity instruction value is 1:2048Usually, the system is designed so that the velocity instruction value will be equal to a product obtained by the multiplication of the resolution of a focus detecting system and a servo gain of a position servo system. Accordingly, for example, if the resolution of the focus detecting system and the gain are 0.01 .mu.m and 10 (1/sec), respectively, the resolution of the velocity instruction value is 0.1 .mu.m/sec and the maximum value of the velocity instruction value is about 205 .mu.m/sec.
In the case of the above condition, if a photosensitive substrate having irregularities of over 80 .mu.rad (0.64 .mu.m/8 mm) is processed, the driving speed in the Z-direction at the pivot reaches the limit of the system, and therefore the interdependence-freeing control which is based on the premise of linear control will break down. As a result, the conventional system suffered from defocus not only in the tilt direction but also in Z-direction, due to the failure of the linear control and this defects would disordered the image focusing.